Fabrication of HR mirror with 90° phase retardance in COIL resonator

With the continuous increase of COIL output power, the conventional HR film stack gradually can't satisfy the practical requirements. In this paper, on the base of multiple frequency film stack, a -200 mm HR phase retardance mirror at 45° incidence is designed and fabricated by ebeam evaporation equipment. The substrate is fused silica, and the high and low refractive index materials are ZrO２ and SiO２ respectively. The measured results show that the reflectivity at 632.8 nm is no less than 95.0%, the reflectivity at 1 315 nm is no less than 99.8%， and phase retardance at 1 315 nm is 90.235°～95.586°．